This invention relates to the art of forming powder images and, more particularly, to heat and pressure belt fuser apparatus for fusing toner images to transparencies.
In the art of xerography or other similar image reproducing arts, a latent electrostatic image is formed on a charge-retentive surface which may comprise a photoconductor which generally comprises a photoconductive insulating material adhered to a conductive backing. When the image is formed on a photoconductor, the photoconductor is first provided with a uniform charge after which it is exposed to a light image of an original document to be reproduced. The latent electrostatic images, thus formed, are rendered visible by applying any one of numerous pigmented resins specifically designed-for this purpose.
It should be understood that for the purposes of the present invention the latent electrostatic image may be formed by means other than by the exposure of an electrostatically charged photosensitive member to a light image of an original document. For example, the latent electrostatic image may be generated from information electronically stored or generated, and this information in digital form may be converted to alphanumeric images by image generation electronics and optics. However, such image generation electronic and optic devices form no part of the present invention.
When a reusable photoconductive surface is utilized, the pigmented resin, more commonly referred to as toner which forms the visible images is transferred to a substrate such as plain paper. After transfer the images are made to adhere to the substrate using a fuser apparatus.
To date the most widely accepted approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with a release layer such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retrained by the heated fuser roller and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate.
To obviate the foregoing toner offset problem it has been common practice to utilize toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied to the fuser roll surface to a thickness of the order of about 0.2 micron to act as a toner release material. These materials provide a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from adhering to the fuser roll surface.
Heat and pressure roll fuser systems used for fusing toner images on various substrates have proven to be unsatisfactory for fusing on smooth transparent substrates such as Mylar, particularly when the toner comprises colored thermoelectric particles. This is because of the low projection efficiency of transparencies fused in this manner. One reason for this may be attributed to the fact that stripping toner images from a hot fuser roll imparts toner layer surface texture. For black images projection efficiency is not a factor, however, for colored images, a rough surface texture imparts optical diffusivity to the images leading to a low projection efficiency and resultant lack of color clarity. For roll fuser systems there is an upper temperature limit for precluding "hot-offset" due to the toner becoming molten instead of just tacky as required in order to preclude "hot-offset".
The projection efficiency of transparencies has been improved using an off-line fuser or transparency finisher marketed by the Canon corporation the transparency or other substrate to be finished or "glossed" is placed in a smooth clear plastic envelope and then fed through a pair of heated rollers. The toner melts and cools against the envelope. The envelope is then peeled back to retrieve the transparency or copy.
Another known method of improving the gloss of color xerographic images on a transparent substrate comprises refusing the color images. Such a process was observed at a NOMDA trade show in 1985 at a Panasonic exhibit. The process exhibited was carried out using an off-line transparency fuser, available from Panasonic as model FA-F100, in connection with a color xerographic copier which was utilized for creating multi-color toner images on a transparent substrate for the purpose of producing colored slides. Since the finished image from the color copier was not really suitable for projection, it was refused using the aforementioned off-line refuser. To implement the process, the transparency is placed in a holder intermediate which consists of a clear relatively thin sheet of plastic and a more sturdy support. The holder is used for transporting the imaged transparency through the off-line refuser. The thin clear sheet is laid on top of the toner layer on the transparency. After passing out of the refuser, the transparency is removed from the holder. This process resulted in an attractive high gloss image useful in image projectors. However, the gloss is image-dependent. Thus, the gloss is high in areas of high toner density because the toner refuses in contact with the smooth plastic sheet and takes on that surface smoothness. In areas where there is little or no toner the gloss is only that of the substrate. The refuser was also used during the exhibit for refusing color images on paper.
As will be appreciated, it is desirable to fuse transparencies such that projection efficiency is optimized without the need to resort to off-line devices for that purpose.
Certain publications and patent applications noted as being possibly relevant to certain aspects of the present invention will now be discussed.
U.S. patent application Ser. No. 08/168,835 filed on Dec. 16, 1993 which is assigned to the same assignee as the instant invention relates to a belt fuser wherein three fuser rollers cooperate with a pressure roller to form an extended fusing zone through which a substrate carrying toner images passes with the toner images contacting fusing belt. Electrical power is applied to the three contact rolls in such a manner that only the portions of the belt in the fusing zone are heated. Thus, the energy is concentrated only in the part of the fusing belt where it is needed for fusing the toner images on the final substrate. Thus, the free extent of the belt or in other words the portion of the belt outside of the fusing zone remains unheated.
To ensure good electrical contact in the presence of silicone oil contamination on the electrically resistive inner surface of the fusing belt, the contact rollers are textured by knurling, bead blasting or other suitable means.
U.S. patent application Ser. No. 08/169,836 no Dec. 16, 1993 which is assigned to the same assignee as the instant invention relates to belt fuser wherein three fuser rollers cooperate with a pressure roller to form an extended fusing zone through which a substrate carrying toner images passes with the toner images contacting a fusing belt. Electrical power is applied to the three fuser rolls in such a manner that only the portions of the belt between the rollers are heated to a predetermined operating temperature in accordance with a setpoint temperature. The free extent of the belt or in other words the portion of the belt outside of the fusing zone is adapted to be heated to various operating temperatures in order to produce prints with different gloss as desired.
U.S. patent application Ser. No. 08/169,838 filed on Dec. 16, 1993 which is assigned to the same assignee as the instant invention relates to a power controller, which does not rely on the use of sensors such as thermistors to control the operating temperature of a belt fuser. It features various preset inputs to control: steady state watts/in, cold start boost watts/in, warmup and cooldown time constants.
The controller sets the desired power based on the on-off cycling of the system. For a cold start, the steady state plus boost power is used, during warmup the boost level is exponentially decreased at a rate set by a warmup time constant. When at rest (with no applied power) the power setpoint is exponentially increased at a rate set by a cool down time constant.
U.S. patent application Ser. No. 08/168,833 filed on Dec. 16, 1993 which is assigned to the same assignee as the instant invention relates to a belt fuser wherein three fuser rollers cooperate with a pressure roller to form an extended fusing zone through which a substrate carrying toner images passes with the toner images contacting fusing belt. Electrical power is applied to the three fuser rolls in such a manner that only the portions of the belt in the fusing zone are heated. Thus, the energy is concentrated only in the part of the fusing belt where it is needed for fusing the toner images on the final substrate. Thus, the free extent of the belt or in other words the portion of the belt outside of the fusing zone remains unheated.
U.S. patent application Ser. No. 08/168,891 on Dec. 16, 1993 which is assigned to the same assignee as the instant invention relates to belt fuser wherein three fuser rollers cooperate with a pressure roller to form an extended fusing zone through which a substrate carrying toner images passes with the toner images contacting fusing belt. Electrical power is applied to the three fuser rolls in such a manner that only the portions of the belt in the fusing zone are heated. Thus, the energy is concentrated only in the part of the fusing belt where it is needed for fusing the toner images on the final substrate. The free extent of the belt or in other words the portion of the belt outside of the three rollers remains unheated. Toner images are directly formed on or transferred to the unheated portion of the fusing belt. The images carried by the belt are then moved through the fusing zone where the images are simultaneously fused and transferred to a final substrate.
U.S. Pat. No. 4,565,439 granted to Scott D. Reynolds on Jan. 21, 1986 relates to a belt fuser for fusing toner images. The fusing apparatus is characterized by the separation of the heat and pressure functions such that the heat and pressure are effected at different locations on a thin flexible belt forming the toner contacting surface. A pressure roll cooperates with a non-rotating mandrel to form a nip through which the belt and copy substrate pass simultaneously. The belt is heated such that by the time it passes through the nip its temperature together with the applied pressure is sufficient for fusing the toner images passing therethrough. The non-rotating mandrel is adapted to having its axis skewed relative to the axis of the pressure roll. A pair of edge sensors are provided for activating a mandrel skewing mechanism. Skewing of the mandrel by such mechanism effects proper belt tracking.
U.S. Pat. No. 4,563,073 granted to Scott D. Reynolds on Jan. 7, 1986 relates to a low mass heat and pressure fuser and release agent management system therefor.
U.S. Pat. No. 5,084,738 granted to Noriyoshi Ishikawa on Jan. 28, 1992 discloses a fusing apparatus having an electrically conductive film which moves in contact with a recording material to which a toner image has been transferred, a pressing roller for causing the film to be brought into contact with the recording material and a plurality of electrodes disposed along a nip between the film and the pressing roller at a position opposing this pressing roller. The electrically conductive film heats up substantially only in the nip as the result of an electrical conductance to this electrode. The toner image on the recording material is heated and fixed by the heat generated in the electrically conductive film positioned in the nip. In a modified embodiment of the foregoing fusing device, a fusing film is fabricated using a thin-film conductive layer made by aluminum deposition or the like. The conductive layer is disposed on the side of a base film comprising carbon black added to a polycarbonate that will contact the transfer material on which a picture image is carried. Power is supplied between a first electrode and a second electrode. Joule heat is produced in the thickness direction of the fusing film.
U.S. Pat. No. 5,182,606 granted on Jan. 26, 1993 discloses an image fusing apparatus including a heater; a film movable with a recording material, in which the recording material has a toner image thereon which is heated through the film by heat from the heater; and the film has a heat resistive resin base layer containing inorganic electrically insulative filler material and a parting layer containing electrically conductive filler material.